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The challenges of responding to these types of changing patterns — not just in Yosemite but across all public lands — are myriad and daunting. One key is to find existing areas in the landscape, such as regions with consistent water resources and biodiversity, that offer benefits across species. Toward that end, Stock, Monahan and other scientists are studying ways to identify and promote “refugia,” which are naturally occurring landscape features that protect or promote not just one but a range of plant and animal species that might be endangered by rising temperature trends, water scarcity or some other stressor.
Land managers could take action to improve refugia for the benefit of target species, such as limiting non-native species to reduce competition for food. They will also study elevation and latitude, as new research shows that due to climate change and habitat loss, up to one-quarter of the total area of the U.S. National Park System is vulnerable to vegetation shifting up slope and northward.
Unfortunately, park managers lack a clear strategy to lessen harm from another change afoot in Yosemite: melting glaciers. Park geologist Greg Stock (Sarah’s husband) says the two remaining glaciers in the park are less than half the size they were back in 1883, the date of their earliest detailed records. One of the two glaciers, the Lyell, has been losing so much ice each year that it no longer moves downhill. The rapidity with which the Lyell is shrinking is important because the glacier contributes to the Tuolumne River headwaters during summer months, well after other sources in the headwaters grow dry.
“In dry years like this, there is very little water in the Tuolumne River except for at the edge of the glacier — it is a reliable source of water,” Greg Stock explained. It’s not just wildlife that relies on those headwaters during late summer months. The flow is also important to the many backpackers who trek into Yosemite’s famous Tuolumne Meadows.
“We’re attributing the melting glaciers primarily to warming summertime temperatures,” he said. Based on climate models, he thinks both of Yosemite’s remaining glaciers could disappear within 50 years.
Yosemite National Park is celebrating its 150th birthday this year. On its 200th birthday, the Half Dome and El Capitan peaks — features formed, Stock noted, by glaciers — will still make Yosemite a majestic, awe-inspiring valley. But the toll another half-century of extreme heat and diminishing snowpacks could take on the park’s ecosystems is unknown.
Both Sarah and Greg Stock field more climate-related questions from the visitors to the park today, compared with when they started working here eight years ago.
“Sometimes people say, ‘Well, yeah, the glaciers are retreating. So what? They grow and shrink with time naturally,’” said Greg Stock. “And that is true, of course. There are no longer 2,000 feet of [glacial] ice in Yosemite Valley. But those cycles of advance and retreat, in that past, had been driven by a certain set of things. We’ve added [greenhouse gas emissions, chiefly carbon dioxide] to that. There is not a big leap of logic to connect the higher CO2 in our atmosphere with the shrinking of these glaciers.”
Impacts such as owl-habitat loss or fluctuating songbird populations are likely hard for park visitors to internalize, but melting glaciers — even though they’re far in the backcountry and invisible to most visitors — is something visitors respond to viscerally.
Sarah Stock said talking to park visitors about climate is important, even when debates emerge. “Each person can have their own interpretation, but mine is based on what we’re seeing with data. The data are right,” she said with a smile.